In recent years, technique for forming a thin film transistor (hereinafter also referred to as TFT) on a basal plate has greatly progressed. Particularly, its application to a driving element of an active matrix type large screen display has been developed. A TFT, which has currently been put to practical use, is manufactured employing a Si based inorganic material such as a-Si or poly-Si. However, the manufacture of a TFT employing such an inorganic material requires a vacuum process or high temperature process, which has a great influence on the manufacture cost.
In order to solve the problem as described above, a TFT employing an organic material (hereinafter also referred to as organic TFT) has been intensively investigated in recent years. Organic materials have a wide choice of materials as compared to inorganic materials. Further, in the manufacturing process of an organic TFT, a process such as printing or coating, which is excellent in productivity, is used instead of the vacuum process or the high temperature process as described above, whereby the manufacturing cost can be suppressed. Further, an organic TFT can be formed on a less heat resistant basal plate such as a plastic film basal plate and its application to many fields has been investigated.
As a coating method of an organic semiconductor material, there is known a liquid droplet coating technique such as an ink jet process or a dispenser method, in which direct coating of a solution containing an organic semiconductor material (hereinafter also referred to as ink) is carried out. There are advantages in these techniques that (1) a vacuum process is not required, (2) waste of materials is reduced, and (3) since direct patterning is possible, an etching process as carried out in photolithography is unnecessary. These techniques can suppress the manufacturing cost, and have been intensively investigated in various fields.
In order to obtain excellent electric properties and high reliability in such an organic TFT, it is necessary that an organic semiconductor layer be formed in an appropriate thickness and precisely at a pre-determined position. However, when an organic semiconductor layer of an organic TFT is formed using the ink jet process or the dispenser method as described above, in some cases a jetted ink wet-spreads on a basal plate due to the influence of surface conditions of the basal plate, ambient atmosphere or the like, and reaches an unnecessary area before the ink is dried to solidify. In such a case, there occurs problem that pattern defect is produced, a sufficient thickness is not obtained, and as a result, an organic TFT with good characteristics cannot be obtained.
In order to solve the problem above, various techniques have been investigated. For example, a technique is known in which a partition wall called a bank is formed around an area to be coated so that the wall prevents jetted ink from flowing outside the area to be coated (Patent Document 1).
However, the effect of preventing jetted ink from flowing outside the area to be coated is not sufficient in the bank disclosed in Patent Document 1. The reason is as follows.
1. Ink Solvent
An organic semiconductor material is ordinarily low in solubility, and even a precursor material, in which solubility is increased, is soluble only in an organic solvent. The organic solvent is poor in the intermolecular interaction and generally low in the surface tension, as compared to water (73 mN/m). The surface tension of an organic solvent is as follows, for example, methanol: 23 mN/m; ethanol: 23 mN/m; isopropyl alcohol: 21 mN/m; acetone: 23 mN/m; benzene: 29 mN/m; n-hexane: 18 mN/m; n-pentane: 16 mN/m; monomethyl ether acetate (PGMEA): 24 mN/m; and anisole: 33 mN/m
Accordingly, ink of an organic semiconductive material employing an organic solvent is likely to wet the bank, and is difficult to control its flow.
2. Ink Concentration
Solubility of an organic semiconductive material is ordinarily less than 1%, and at most around several percent. Therefore, in order to secure a required thickness of an organic semiconductor layer it is necessary to coat a large amount of ink. Consequently, it is not easy to prevent the ink flow via a bank.
3. Ink Coating Region
A channel, where an organic semiconductor layer is to be formed, has at most a size of 30 μm (short side)×100 μm (long side), and ordinarily a size of 10 μm (short side)×30 μm (long side). On the other hand, the size of ink droplets jetted by means of an ink jet process is 10 to 20 μm Φ. Therefore, when deposition accuracy of the ink droplets or pattern forming accuracy of the channel is taken into account, the ink droplets, after deposition to the channel, spread on the bank. That is, it is difficult to prevent the undesired ink flow by means of a bank.
In order to solve the above problem, a method is known in which the shape of the bank is improved and an opening is formed both on the channel and on an ink guide region extending outward from the channel, whereby the ink is introduced into the channel through the ink guide region (Patent Document 2).